Structural colors in nature, such as those on butterfly wings, beetle cuticles and peacock feathers, have attracted considerable attention in a variety of research areas. Structural color has many characteristics that differ from those of chemical pigments or dyes. For example, in the feathers of a peacock, various colors result from the interaction of light with a single biological material: melanin rods. The iridescent colors are formed as a result of the lattice spacing of the rods. In nature, a single biological material with different physical configurations displays various colors, which greatly simplifies the manufacturing process in producing multiple colors. The unique colors originating from the physical structures are iridescent and metallic, and cannot be mimicked by chemical dyes or pigments. Furthermore, structural color is free from photobleaching, unlike traditional pigments or dyes.
Owing to its unique characteristics, there have been many attempts to make artificial structural color through various technological approaches such as colloidal crystallization, dielectric layer stacking and direct lithographic patterning. The colloidal crystallization technique is most frequently used to make a photonic crystal, which blocks a specific wavelength of light in the crystal and therefore displays the corresponding color. Gravitational force, centrifugal force, hydrodynamic flow, electrophoretic deposition and capillary force from the evaporation of solvents are used to assemble the colloidal crystals. Although these methods produce structural colors with a large area, the growth of colloidal crystals usually takes a long time so as to achieve better crystallization and fewer defects. Also, because the bandgap of a photonic crystal is dependent on the size of the colloids used, different sizes of colloidal suspensions are needed to produce multicolored structures. Furthermore, there have been great technological difficulties in assembling colloids of different sizes to form these multicolored patterns with fine resolutions. Dielectric layer stacking and lithographic patterning of periodic dielectric materials generates structural color by directly controlling the submicrometer structure of the surface. Various fabrication processes have been reported, including replicating natural substrates, depositing materials layer by layer and etching a substrate using various lithographic techniques. These approaches are advantageous in that they accurately fabricate a periodic dielectric structure on the surface, which controls the desired photonic bandgap. However, in spite of the advantage of sculpting sophisticated nanostructures in a well controlled manner, a cost-effective manufacturing scheme to generate multicolored structures over a large area is hard to achieve owing to the requirement for a vacuum process. Moreover, great effort and long process times are necessary to produce multicolored patterns on a substrate, because different pitches of dielectric stacks are required to achieve different colors.